In many areas of technology, it is desirable to control the spatial distribution of magnetic fields carefully. Well-controlled magnetic fields are particularly important in nuclear magnetic resonance (NMR) spectroscopy and other magnetic resonance (MR) applications. In many NMR spectroscopy experiments, a strong, static magnetic field is applied in a region of space that contains a sample under study, and it is desirable that this field be as spatially uniform as possible in order to observe important but subtle variations in the magnetic response of the sample. It is also desirable in many NMR applications to have a static magnetic field that is as strong as is practical.
At least three classes of magnets have been used to provide a strong, static magnetic field in NMR devices: superconducting electromagnets, resistive electromagnets, and permanents magnets. Permanent magnets or arrays thereof can be advantageous in applications where low cost, low maintenance or portability are desirable.
In practice, permanent magnets are often accompanied by pole pieces. The term “pole piece” is more fully defined herein but generally refers to a piece of magnetically permeable material placed in the vicinity of magnets in order to contribute to or shape a magnetic field.